This invention relates to a cooling device for the wheel of a continuous casting machine of the wheel and belt type, comprising an annular header disposed internally to the casting ring of the wheel and a plurality of spray nozzles arranged on the header for spraying jets of cooling fluid onto the casting ring.
The performance limits of a continuous casting machine in terms of production rate and metal quality are determined by the characteristics of the cooling system.
In a continuous casting machine of the wheel and band type, an ingot of indefinite length is produced by feeding the molten metal into the mould defined by the channelled ring of the casting wheel and the metal band which embraces said ring through a certain arc, cooling the metal during the wheel rotation and extracting it from the channel when it has reached a sufficient degree of cohesion. It is therefore necessary for the metal to reach a sufficiently low temperature, i.e. a degree of cohesion sufficient for extracting the ingot from the channel without breakage or dangerous bending, within less than one revolution of the wheel, so that the channel is ready to receive new molten metal.
The cooling rate, i.e. the quantity of heat which the cooling fluid is able to remove from the metal in unit time, imposes a maximum value on the rotational speed of the wheel, which is the parameter which determines the production rate.
Thus to attain high production rates, the heat exchange between the solidifying metal and the cooling fluid must be raised to a maximum.
As already stated, a further problem which depends on the characteristics of the cooling system is the quality optimisation of the metal produced. In this respect, the metal must pass from the liquid state to the solid state not only in the shortest possible time, but also in a regular and uniform manner, as the manner in which this passage of state takes place considerably influences the characteristics of the internal structure of the metal and therefore its mechanical and electrical characteristics. A cooling process of maximum rapidity, as would be desirable for increasing the production rate, may however have negative effects on the passage of state of the metal in the sense of not allowing the formation of the optimum crystalline structure.
Thus the efficient, properly gauged and most of all uniform cooling of the inner surface of the ring and metal band is essential in order to obtain an optimum metal quality and a high production rate.
The attempts made up to the present time to obtain rapid and uniform cooling in a casting wheel comprise numerous methods.
The most conventional method for cooling the ring of a casting wheel and the metal contained in it comprises passing cooling fluid into ducts of small cross-section formed in the bulk of the ring. This method creates a serious problem in the attempt to obtain a high heat transfer rate between the ring and cooling fluid. This is because the temperature of the ring, particularly in the zone where the molten metal is received, may reach such high values that the layer of coolant adhering to the surfaces of the ducts may vaporise, so considerably increasing the resistance to heat passage between the walls of the duct and the coolant, and therefore the heat exchange. Furthermore, the fact that the ducts are divided into groups along the periphery of the ring leads to non-uniform cooling, resulting in the said crystallization defects, and the prevention of a high cooling rate to the detriment both of the production rate and the ring life.
Attempts to overcome these disadvantages have led to the construction of a device for continuously feeding jets of coolant onto the inner surface of the casting ring. This device consists of a non-rotating annular header, concentric and internal to the ring, and comprising on the surface facing the ring a series of spray nozzles of adjustable flow rate. Cooling of good uniformity can be obtained with this device, if the jets are carefully regulated. However this system also has limitations.
In these devices, because of the closeness of the spray nozzles, as is required to obtain the most uniform cooling possible, and the turbulence created by the impact of the jets on the ring surface, there is a partial permanence of hot fluid on the ring inner surface. Even though this hot fluid becomes partially replaced by cold fluid, the phenomenon is continuous and this leads to a considerable limitation in the efficiency and rapidity of heat transfer, which limits the maximum allowable rotational speed of the casting wheel and hence its production rate. Furthermore this phenomenon can lead to a certain non-uniformity of cooling of the ring surface, which negatively influences the internal metal structure.